1. Field of the Invention
This invention relates to a bicycle multiple sprocket assembly having at least two diametrically different sprockets. More specifically, the invention relates to a multiple sprocket assembly which is designed to provide smooth chain shifting from a smaller sprocket to a larger sprocket.
The term "multiple sprocket assembly" as used herein includes a multiple chainwheel (front gear) which is mounted on a pedal crank assembly, and a multiple freewheel (rear gear) mounted on a rear wheel hub.
2. Description of the Prior Art
Many of currently available bicycles not only have a multiple freewheel to constitute a rear gear mounted on a rear wheel hub, but also incorporate a multiple chainwheel to constitute a front gear mounted on a pedal crank, thereby increasing the number of selectable speeds. A selected sprocket of the front gear is connected to a selected sprocket of the rear gear by an endless chain, and the pedalling force applied to the front gear is transmitted through the chain to the rear gear for driving the bicycle forward.
With a multiple sprocket assembly, a speed change is performed by causing a derailleur (front derailleur or rear derailleur) to laterally press a portion of the chain entering to the sprocket assembly in rotation, which results in that the chain is laterally inclined for shifting from a smaller sprocket to a larger sprocket or vice versa. Obviously, the speed change performance of the sprocket assembly is determined by the smoothness and promptness in disengaging the chain from a presently engaging sprocket for engagement with a target sprocket.
In shifting the chain from a larger sprocket to a smaller sprocket, the chain need only be disengaged from the larger sprocket because a tension applied to the chain by the return spring of the rear derailleur can be utilized for causing the chain to automatically fall into engagement with the smaller sprocket. In shifting the chain from the smaller sprocket to the larger sprocket, on the other hand, mere disengagement of the chain from the smaller sprocket is not sufficient because the chain must be radially lifted against the chain tension with an additional energy to a height suitable for engagement with the larger sprocket. Thus, it has been proven more difficult to shift the chain from the smaller sprocket to the larger sprocket than shifting it from the larger sprocket to the smaller sprocket. Particularly, difficulty arises when the diametrical difference between the two sprockets is small because, in this case, the chain must incline laterally with a large angle (i.e., with a large stress) in a relatively short chain shifting path.
Various attempts have been made to improve chain shiftability from a smaller sprocket to a larger sprocket. However, none of the past attempts have produced satisfactorily successful results as yet.